If money buys happiness, the Giants and their fans ought to be delirious. If huge cash payouts equate to success, the Giants and their fans should buckle up and prepare for quite a ride.

Co-owner John Mara, following another 6-10 season, decided enough was enough with coach Tom Coughlin and stated “Jerry knows it’s on him,’’ referring to general manager, Jerry Reese. Well, Reese headed into NFL free agency checkbook in hand, pen at the ready and embarked on a frenzied, unprecedented spending spree.

Reese and the Giants broke with their longstanding tradition and went absolutely bonkers, throwing money around as if it was confetti dropping down from the Canyon of Heroes. Their breathtaking frenzy Wednesday culminated with the stunning signing of defensive end Olivier Vernon, who comes aboard on a five-year contract worth $85 million, including a staggering $52.5 million in guaranteed money — the most the Giants ever have handed out in franchise history.

The investments are in youth — none of the three newcomers is older than 27 — and durability. Vernon, Jenkins and Harrison have missed a total of four games due to injury in their careers — all by Jenkins.

The Giants headed into free agency $56.6 million under the cap and Vernon nearly broke the bank with the richest deal in NFL history for a defensive end — more than J.J. Watt — as far as guaranteed money and per year average ($17 million). He will get $29 million in 2016 and $54 million in the first three years.

The Giants view Vernon, 25, as a young star-in-the-making, a clean player with no medical history. Vernon has 29 career sacks and is coming off his finest season, but he became expendable after the Dolphins signed former Bills defensive end Mario Williams to a two-year, $17 million deal.

The Dolphins, after signing Williams, withdrew the transition tag of $12.7 million they placed on Vernon, making him an unrestricted free agent and allowing him to become a big-ticket winner on the open market.

After re-signing Pierre-Paul (one year, $10.5 million) on Tuesday, the Giants came to terms on a massive five-year, $62.5 million contract with Jenkins and secured Harrison with a five-year, $46.25 million deal. The addition of the 350-pound Harrison gives the Giants one of the league’s top run-stoppers to go along with Pierre-Paul, Vernon and holdover Johnathan Hankins on a revamped and potentially potent defensive line.

On Monday, Jenkins and Darrelle Revis worked out together in Hollywood, Fla. Two days later, Jenkins, 27, received $29 million in guaranteed money, making him the second highest-paid cornerback in the league, behind only Revis. Both Revis and Jenkins are represented by Neil Schwartz and Jonathan Feinsod.

This signals the end of the Giants career of Prince Amukamara, who was not made a priority to re-sign. Jenkins will step into Amukamara’s role as a starter alongside Dominique Rodgers-Cromartie, giving the Giants a pair of playmaking corners. The plan is for Jenkins, Vernon and Harrison to take physicals Thursday morning then sign their contracts at the Giants’ facility.

The Rams wanted to keep Jenkins, but on Twitter he scoffed at their offer (five years, $45 million), calling it “total disrespect when u held down one side all 4 years,’’ adding the hashtag “#ThanksButNoThanks.’’

As a player, Jenkins is dynamic, but at times a bit reckless. His six touchdown returns (five interceptions, one fumble recovery) since 2012 are the most in the NFL in that span.

Yet the top analytics site Pro Football Focus calls the Jenkins signing a “boom or bust move for the Giants,’’ citing Jenkins’ 22 touchdowns allowed, the third-highest total for a cornerback the past four years. Jenkins was kicked off the team at the University of Florida for a second failed drug test for marijuana and played his senior year at North Alabama.

Harrison, 27, started every game the past three years for the Jets. Pro Football Focus ranked him as its No. 1 run defender in the league in 2015 and graded him with the most run stops in the NFL the past three years.